The present invention relates to the field of data compression, and more particularly to data compression on a digital subscriber line (DSL) loop.
With the increasing popularity of the Internet, there has been a corresponding increase in the demand for high rate digital transmission over the local subscriber loops of telephone companies. A loop is a twisted-pair copper telephone line coupling a user or subscriber telephone to a central office (CO).
Traditionally, data communication equipment uses the voice band of the subscriber loop. Such equipment includes voice band modems, which operate at up to 56 kbps using compression techniques. On the other hand, Integrated Services Digital Network (ISDN) systems have boosted data rates over existing copper phone lines to 120 kbps. However, traditional voice band equipment is limited by the maximum data rate of the existing switching networks and Pulse Code Modulation (PCM) data highways.
Utilizing the frequency bandwidth of the loop outside the voiceband has enabled other high-speed systems to evolve. However, because loops can differ in distance, diameter, age and transmission characteristics depending on the network, they pose some limitations and challenges for designers of these high-speed systems.
Current high-speed digital transmission systems of the above type include asymmetric, symmetric, high-rate, and very high-rate digital subscriber loops, conventionally known as ADSL, SDSL, HDSL and VDSL, respectively. Normally these and other similar protocols are known as xDSL protocols.
Of these flavors of xDSL, ADSL is intended to co-exist with traditional voice services by using different frequency spectra on the loop. An overview of ADSL is provided in the ADSL Forum""s Technical Report TR-017, xe2x80x9cATM Over ADSL Recommendationxe2x80x9d (March 1999), which is incorporated herein by reference for all purposes. More detail on ADSL can be found in the document ANSI T1.413, xe2x80x9cAsymmetric Digital Subscriber Line (ADSL) Metallic Interfacexe2x80x9d (1998), which is incorporated herein by reference for all purposes.
In the future, it is possible that multiple different transmission schemes may be employed in different frequency bands on the same loop, and that these transmission schemes may include traditional analog voice services as well as current and new forms of xDSL. In today""s ADSL systems, the plain old telephone services (POTS) use the frequency spectrum between 0 and 4 kHz and the ADSL uses the frequency spectrum between 30 kHz and 1.1 MHz for data over the telephone line. ADSL partitions its frequency spectrum with upstream (subscriber to CO) transmission in a lower frequency band, typically 30 kHz to 138 kHz, and with downstream transmission in a higher frequency band, typically 138 kHz to 550 kHz or 1.1 MHz. ADSL uses a discrete multi-tone (DMT) multi-carrier technique that divides the available bandwidth into approximately 4 kHz sub-channels.
In order to maximize the throughput on a given channel, it is important to minimize the redundancy in the transmitted data, followed by the careful addition of some redundancy (in order to enable the use of forward error correction (FEC)). Thus far, there has been a lot of activity to improve the performance of DSL (particularly on long loops) with the use of better FEC. Reed-Solomon encoding and Trellis Coded Modulation are already part of the G.DMT specification for ADSL, and further additions of concatenated convolutional and turbo codes are open issues for the G.DMT specification.
It is appropriate to consider first whether data compression should be performed at the physical layer in an ADSL modem. Performing data compression at the physical layer for a DSL link is practical. If performed at higher layer protocols and applications, it is difficult to ensure that the union of all application programs, operating systems, network protocols, and content providers would present data to the ADSL link in a compressed format. Some users of some applications would use data compression, while many users of the Internet would continue to transfer files, download web pages, and exchange email without the benefit of compression.
The inclusion of data compression in the DSL link does no harm, but it potentially provides a great benefit. It is possible to design a data compression scheme that will not provide a degradation of throughput.
Finally, DSL provides data rates and services that are quite different from dialup modems and Ethernet-based local area networks (LANs). DSL specific issues need to be addressed by DSL standards and cannot be left to general networking solutions. One example of a DSL specific issue is the use of ATM cells over the link. It is possible to exploit the redundancy in the ATM cell headers to help compress the traffic on the loop by up to 10%. Another example is the multiplicity of services that can be run on the bandwidth of a DSL link. It is possible to download a file while browsing the web and listening to an audio broadcast. Compression algorithms in such an environment need to be agile to the interleaved traffic.
The present invention provides for the use of compression on the data in a channel to remove some of the inherent redundancy, in order to yield much better throughput, particularly in conjunction with certain powerful FEC schemes. At the transmission end, an ADSL termination unit includes a descrambler and a compressor between its scrambler and interleaver. In this manner the compression may be performed on unscrambled data that has a higher redundancy than scrambled data, thereby improving compression. At the reception end, the ADSL termination unit includes a decompressor and a scrambler between its deinterleaver and descrambler.
The removal of redundancy on the transmitted data is independent of the data rate on the loop. However, for long loops with reduced data rates, the improvement can provide significant benefits to the end user of the DSL service.
According to one embodiment of the present invention, a compression system for ADSL includes a central office ADSL termination unit (ATU-C) and a remote ADSL termination unit (ATU-R). The ATU-C includes a descrambler and a compressor between its scrambler and interleaver. The ATU-R includes a decompressor and a scrambler between its deinterleaver and descrambler. In this manner, compression may be performed on unscrambled data that has a higher redundancy than scrambled data, thereby improving compression.
According to another embodiment of the present invention, a transmitting ATU includes an asynchronous transfer mode (ATM) transmission convergence circuit, a scrambler circuit, a descrambler circuit, a compression circuit, and an interleaver circuit. The descrambler circuit and compression circuit operate as described above regarding the compression system.
According to yet another embodiment of the present invention, a receiving ATU includes a deinterleaver circuit, a decompression circuit, a scrambler circuit, a descrambler circuit, and an ATM transmission convergence circuit. The scrambler circuit and decompression circuit operate to decompress the compressed data received from the above-described transmitting ATU.
One aspect of the invention provides for the limiting of the maximum compression bandwidth to assist ATM provisioning.
Another aspect of the invention provides for limiting the average compression bandwidth to assist ATM provisioning.
A still further aspect of the invention provides for ATM flow control over the ADSL loop to assist ATM provisioning in the presence of bandwidth variation. (Compression is one way to get bandwidth variation, among others.)
A still further aspect of the invention provides for the use of multiple or hybrid compression algorithms to match the interleaved data traffic seen on ADSL loops.